Control of Subduction Zone Age and Size on Flat Slab Subduction

Abstract

Flat slab subduction is an enigmatic style of subduction where the slab attains a horizontal orientation for up to several hundred kilometers below the base of the overriding plate. It has been linked to the subduction of buoyant aseismic ridges or plateaus, but the spatial correlation is problematic, as there are subducting aseismic ridges and plateaus that do not produce a flat slab, most notably in the Western Pacific, and there are flat slabs without an aseismic ridge or plateau. In this paper an alternative hypothesis is investigated which poses that flat slab subduction is associated with subduction zones that are both old (active for a long time) and wide (large trenchparallel extent). A global subduction zone compilation is presented showing that flat slabs preferentially occur at old (>∼80-100 Myr) and wide (≥∼6000 km) subduction zones. This is explained by the tendency for wide subduction zones to decrease their dip angle in the uppermost mantle with progressive time, especially in the center. A set of numerical subduction models confirms this behavior, showing that only the central parts of wide slabs progressively reduce their slab dip, such that slab flattening, and ultimately flat slab subduction, can occur. The models further show that a progressive decrease in slab dip angle for wide slabs leads to increased vertical deviatoric tensional stresses at the top surface of the slab (mantle wedge suction), facilitating flat slab subduction, while narrow slabs retain steep dip angles and low vertical deviatoric tensional stresses. The results provide a potential explanation why present-day flat slabs only occur in the Eastern Pacific, as only here subduction zones were old and wide enough to initiate flat slab subduction, and why Laramide flat slab subduction and South China flat slab subduction were possible in the geological past.

The role of slab geometry in the exhumation of cordilleran-type orogens and their forelands: Insights from northern Patagonia

“…23 Ma, when the break-up of the oceanic Farallon Plate into Nazca and Cocos Plate reorganized the plate convergence (Somoza, 1998;Lonsdale, 2005). This new configuration allowed an increase of interplate coupling that was not necessarily linked to a change of slab dip (Cerpa et al, 2018;Schellart, 2020). Resumption of the upper plate shortening is recorded along the Patagonian Precordillera and associated with the Miocene rejuvenation of the North Patagonian Batholith (Echaurren et al, 2016).…”

Magmatic record of the Mesozoic geology of Hainan Island and its implications for the Mesozoic tectonomagmatic evolution of SE China: effects of slab geometry and dynamics in continental tectonics

“…14a). A global synthesis and three-dimensional modelling of 17 modern subduction zones and their kinematics suggest that the trench-parallel extent of a subducted slab (its width) may vary between 300 and 7000 km, and that it affects the slab dip angle and subduction partioning along a trench more than the age of the downgoing oceanic lithosphere (Schellart et al 2010;Schellart, 2020). Very large widths of downgoing oceanic plates (8000 to >10 000 km) may resist slab retreat, have fast trench-normal subduction plate velocity and be pushed over by the continental upper plate, causing flat subduction.…”

“…Very large widths of downgoing oceanic plates (8000 to >10 000 km) may resist slab retreat, have fast trench-normal subduction plate velocity and be pushed over by the continental upper plate, causing flat subduction. Conversely, small widths (~1500 km or less) result in slab-buoyancy-driven trench retreat, slow trench-normal subduction plate velocity (and small subduction partitioning -<0.5), and slab steepening with rollback (Schellart et al 2007;Schellart, 2020). The former configuration with flat slab leads into significant crustal shortening in the upper continental plate, as was the case in western North America during the Late Cretaceous -Early Eocene Sevier-Laramide orogeny due to the rapid Farallon plate motion caused by its enormous slab width (>10 000 km; Schellart et al 2010).…”

Subduction zone processes and crustal growth mechanisms at Pacific Rim convergent margins: modern and ancient analogues

“…Wu et al 2019), particularly in the North China Block; in the development of extensive magmatic belts and associated mineralization zones (Deng et al 2017); and in the formation of margin-parallel transform-transcurrent fault systems and kinematically and temporally related terrestrial sedimentary basins (Zhang et al 2017(Zhang et al , 2019b. Changing slab geometries, widths and slab rollback rates, and the amount and compositions of subducted sediments affected the melt evolution and formation of different types of magmatism and mineralization in the continental upper plate (Dilek & Altunkaynak, 2009;Jamali et al 2010;Conticelli et al 2015;Deng et al 2017;Feng et al 2018;Niu X et al 2019;Schellart, 2020). Thus, ESE China (Fig.…”